Conventional golf balls include a core or center region made of one or more compositions having a defined PGA compression value, and a cover made of one or more compositions having another PGA compression value. The combination of these compositions and structures results in a ball having an overall PGA compression.
As a general statement, a lower overall PGA compression value results in greater distance off the club, but reduced ball spin. The loss of spin causes a ball to release around the green, making it more difficult to stop the ball at a desired position.
Ball spin is influenced by, among other things, the cover material. In the past, balata materials were used as a cover material. While balata produced greater spin, it was also subject to cutting and shear if the ball was mis-hit. Moreover, balata is expensive. As a result, balata covered balls were generally preferred by lower handicap players willing to pay more for a golf ball.
More recently, synthetic thermoplastic resins have been developed as an alternative to balata. Most notably these synthetic thermoplastic materials are ionomeric resins. Ionomeric resins are polymers containing interchain ionic bonding. As is well known in the chemical arts, ionomeric resins are generally ionic copolymers of an olefin having from about two to about eight carbon atoms, such as ethylene and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. The pendent ionic groups in the ionomeric resins interact to form ion-rich aggregates contained in a non-polar polymer matrix. Metal ions, such as sodium, zinc or magnesium are used to neutralize some portion of the acidic groups in the copolymer. This results in a thermoplastic elastomer, which exhibits enhanced flight characteristics and durability when compared to golf balls constructed with balata covers. However, the advantages gained by enhanced durability have been offset by the decreased playability properties.
The most recognizable of these was developed by E. I. Dupont De Nemours & Company as commercially available under the name Surlyn®. There are several different grades of Surlyn® having varing acid content levels that effect the properties in different manners. Ionomeric resins from Exxon under the Trade name Iotek® are also available.
High acid content ionomers, which, for the purposes of this application, generally have an acid content of at greater than 15%, and general produce harder and tougher covers. These high acid resins usually have a flex modulus from greater than about 50,000 PSI. While high acid content ionomeric resins provide for better abrasion and cut resistance, their high flex modulus and hardness, 60 Shore D and higher, produce an uncomfortable hard feel to the golfer and reduces the amount of spin that can be imparted to the ball at impact. Examples of Surlyns®s in this category include grades 8140, 8150, 9120, 9910, 8945, and others.
To alleviate the hard feel associated with high acid ionomer resin covers, Dupont developed a series of low acid ionomeric resins that produce a softer feeling cover. These resins generally have an acid content less than 15%, and have flexural modulus from about 4,000 to 7,000 PSI. These ionomers or blends thereof provide for better feel and spin, and have very similar physical properties to those of balata. However, the low flex modulus and Shore D hardness, e.g. 40 or less, result in covers with poor cut, shear and abrasion resistance. Worse, wound balls with these covers tend to go “out-of-round” quicker than wound balls with balata covers. Examples of Surlyn® resins in this category include grades 6320, 8320, and 9320W, and others. The low modulus ionomers when used without high flexural modulus blends
In an attempt to overcome the negative effects of high and low acid resins, DuPont suggested certain blends of high acid/low acid resins to produce balata-like properties in a golf ball covered with this material. The qualities found in these golf balls, especially as to durability, were not optimal, and while the blends reduced the effects associate with each, processing problems and cost increased, leaving it difficult to manufacture a low cost ball having physical characteristics similar to balata.
Others have suggested various combinations of high acid/low blends. U.S. Pat. No. 4,884,814 to Sullivan teaches blending a hard ionomer resin with a soft ionomer resin to produce a cover having enhanced playability characteristics and a good durability for continuous play. In that reference, a golf ball cover blend includes a hard Surlyn ionomer resin, having a hardness of 60–66 on the Shore D scale as measured in accordance with ASTM method D-2240, and various soft Surlyn ionomer resins having hardness ranging from 25–40 on the Shore D scale. Specifically, the patent claims 25–70% of the hard ionomer which is a sodium or zinc salt of the specified copolymer and having flexural modulus of 30,000–50,000 psi, blended with 75–25% of the soft ionomer which is a sodium or zinc salt of a specified terpolymer and having a flexural modulus of 3000–7000 psi.
U.S. Pat. No. 5,120,791, also to Sullivan, is designated an improvement to the earlier patent and reveals a composition said to have enhanced carrying distance. The blends of this second Sullivan patent include 90–10% of an acrylic acid based hard ionomer neutralized with sodium or zinc and having a Shore D hardness greater than 50 and a flexural modulus up to 70,000 psi, with an acrylic acid based soft ionomer having a Shore D hardness of 20–40 and a flexural modulus of 2000 to 8000 psi.
Covers produced according to these references have improved playability over traditional Surlyn covers but at a cost of losing resilience and distance properties.
More recently, the development of polyurethane covers allow for a low compression ball having a softer cover. Polyurethane cover golf balls provide a lower compression ball with a soft cover that is less susceptible to cutting while maintaining a relatively high spin rate. However, polyurethane balls are also expensive, making them less desirably for higher handicap or beginning golfers.
What is desirable is a relatively inexpensive golf ball having a low compression for distance and a high spin rate. The cover should be softer, but tougher to avoid cutting, shearing and/or abrasion from mis-hits by the beginning or high to mid handicap golfer.